The present invention relates to a method for preparing a silver haloiodide photographic emulsion. More particularly by, the invention concerns a method of preparing a tabular grain silver bromoiodide emulsion.
In photography, especially for color pictures, silver halide emulsions are commonly used that contain a certain proportion of iodide. The silver bromoiodide grains are made up of silver bromide crystals in which the silver iodide can theoretically be incorporated up to the limit of its solubility in the silver bromide, i.e., up to about 40 mole % depending on the temperature at which the grain was formed. In most cases, the percentage of iodide lies between 0.1 and 10 mole %, and more particularly between 0.5 and 5 mole %. The percentages of halide are given relative to the silver present in the emulsion. The percentages of iodide in the silver bromoiodide emulsions are the result of a compromise between the advantages provided by the iodide (better formation of the latent image, better natural speed, better adsorption of additives) and the disadvantages arising from the presence of iodide (inhibition of development, resistance to chemical sensitization). The localization of the iodide in the grain also influences the photographic characteristics of the emulsion. This localization of the iodide in the grain is determined by the conditions of preparation. In general, for silver haloiodide emulsions intended for negative or reverse color photographic products the iodide is added in a range between 60 to 80% of the total precipitation of the emulsion. Silver iodide is much less soluble than silver bromide and silver chloride. Accordingly, it is possible to distribute the iodide throughout the grain using double jet precipitation, where the iodide and the bromide are added together in the reactor at the same time as the silver salt, or using triple jet precipitation, with a silver salt jet and two simultaneous jets of bromide and iodide respectively. However, with this method, it is not possible to form zones within the grain that have high concentrations of iodide. To obtain zones with high iodide concentrations iodide needs to be added while the addition of bromide is stopped or slowed. The iodide can be added in the form of seeds or fine grains of silver iodide, or in a controlled manner as a solution, for example of sodium or potassium iodide. When silver iodide seeds are used these must obviously be prepared and stored separately during use. The introduction of separate soluble iodide solutions during the precipitation has also been tried. When separate iodide solutions are used during precipitation, they have to be introduced into the reactor at low flow rates, less than 20 ml/minute and even less than 10 ml/minute, in order to optimize the formation of silver iodide on the silver halide already precipitated, and prevent the formation of grains with undesired shapes. In addition, if too much iodide is added in a very short time, then the grains may be destroyed. These conditions do not favor an industrial use of this method.
Various methods of the types stated above, designed to incorporate iodide into silver halide grains are described, for example, in U.S. Pat. Nos. 3,206,313; 3,317,322; 3,505,068 and 4,210,450. Some of these references concern tabular grain silver haloiodide emulsions. Tabular silver halide grains are grains that possess two main parallel faces that are appreciably greater in surface area than the other faces of the grain. The tabular grain shape is expressed by the aspect ratio of the grain, which is the ratio of the equivalent circular diameter (ECD) to the thickness (t), or distance between the two main parallel faces. A grain is considered tabular if its aspect ratio (ECD): t is greater than 2. An emulsion is said to be xe2x80x98tabular grainxe2x80x99 when at least 50%, and preferably at least 70% and even at least 90% of the total projected surface of the grains of the emulsion is composed of tabular grains.
Because tabular grain emulsions are more and more widely used in photography, efforts have been made to optimize the introduction of iodide in the emulsions, as described for example in U.S. Pat. Nos. 4,433,048; 5,358,840 and 5,840,475. In another reference (U.S. Pat. No. 5,667,954), a method is described for preparing a silver bromoiodide emulsion with tabular grains whereby a xe2x80x98hostxe2x80x99 emulsion of silver bromide is first precipitated, to which is then added a solution of iodide ions without introducing a silver salt. This method aims to obtain a localization of silver iodide in certain zones of the grain.
However, the methods stated above all present various disadvantages, as indicated, making their industrial application difficult, especially as regards tabular grain emulsions.
Therefore, it is desirable in the art to have a method that makes it possible to overcome these disadvantages.
The present invention provides a method of preparing a silver haloiodide photographic emulsion which allows a robust and efficient way to introduce iodide in the emulsion and to monitor the localization of the iodide in the emulsion. More particularly, the invention provides a method for preparing a tabular grain haloiodide photographic emulsion.
The present invention for preparing a silver haloiodide emulsion, comprises the steps of (i) preparing a silver halide host emulsion in a reactor vessel, and (ii) simultaneously adding jets of soluble silver salt and iodide salt solutions to the reactor vessel to precipitate silver iodide onto the silver halide host emulsion, the jets being added in at least two pulses separated by a pause, where the respective flow rates of the silver salt and the iodide salt solutions added are at least V/Voxc3x97100 ml/minute each during the pulses, wherein Vo is 18 l and V is the total volume of precipitated emulsion in the reactor vessel.
In one aspect of the method of the invention, a part of the iodide in the iodide jet of step (ii) is replaced by bromide during at least one pulse.
In yet another aspect of the invention, the method comprises the steps of:
(i) precipitating a host tabular grain silver bromide, silver chloride, or silver bromochloride emulsion in a reactor vessel, and
(ii) simultaneously adding jets of soluble silver salt and iodide salt solutions to the reactor vessel to precipitate silver iodide onto the host emulsion, the jets being added in at least two pulses separated by a pause, where the silver salt jet and the iodide salt jet have a flow rate of at least V/Voxc3x97100 ml/minute each during the pulses, wherein Vo is 18 l and V is the total volume of precipitated emulsion in the reactor vessel.